President Trump's signature Tuesday on two executive orders clears the way for two pipelines to advance, thrusting the controversial projects back into the spotlight.

Here's a look at the two projects:

Dakota Access Pipeline

What it is: The 1,172-mile pipeline project would run from the Bakken oil fields in northwest North Dakota through South Dakota, Iowa and end in Illinois. It would carry 570,000 barrels of crude oil per day and cost $3.78 billion.

How it got started: In December 2014, Energy Transfer Partners LP applied to build the pipeline. On Jan. 25, 2016, Dakota Access LLC, a subsidiary of Energy Transfer, said it received a green light from the North Dakota Public Service Commission after gaining approval from three of the states involved. In March, Iowa became the fourth state to approve the plan. In July, the U.S. Army Corps of Engineers approved the final land easements and water crossing, allowing the project to move ahead.

Why it's controversial: The pipeline has been under fire by Native Americans, who are concerned about it being built on sacred ground and burial sites. Among the waters granted easements are the Mississippi River, Lake Sakakawea and Lake Oahe, a sacred site for the Standing Rock Sioux. In July, the tribe sued the Army Corps of Engineers, seeking an emergency halt to all construction. Greenpeace and other conservation groups have also opposed the project over concerns about impacts on air, water, wildlife and farming.

What supporters say: Supporters say the pipeline is the safest and most environmentally sensitive way to transport crude oil from domestic wells to U.S. consumers. They also say the pipeline will not cross Standing Rock Sioux reservation land, even at the point of dispute at Lake Oahe. Developers point to the economic benefits from jobs and sales and income taxes.

What you might remember: Last spring the Standing Rock Sioux set up a camp within the reservation as a base of protest. In the fall the protests escalated as thousands jammed the sites and protesters clashed with police. On Nov. 15, protests were held in major U.S. cities on a "National Day of Action." On Dec. 4, cheers erupted at the Oceti Sakowin Camp on news that the Corps of Engineers denied an easement that would have allowed the pipeline to cross Lake Oahe — effectively halting work on the project.

Keystone Pipeline

What it is: The proposed 1,179-mile cross-border pipeline would travel from Alberta, Canada, to Nebraska, where it would connect with an existing pipeline. It would carry an estimated 830,000 barrels of oil each day.

How it got started: The pipeline was commissioned in 2010 by Canada's energy commission to run from Alberta to refineries in Illinois and Texas. It is being built in four phases, three of which are in operation. Developers say the first three phrases have the capacity to deliver about 1 million barrels of oil a day.

Why it's controversial: It is the fourth phase of the project — Keystone XL — that has been in dispute in the U.S., with Nebraska at ground zero. The proposed path has the pipeline running over the Ogallala Aquifer, one of the world's largest aquifers, which supplies drinking water to more than 2 million people in the Great Plains. Environmentalists fear leaks could contaminate the aquifer. Scientists have also expressed concern about greenhouse gas emissions. Others say the project is an "export pipeline" that would send Canadian oil to overseas markets without benefiting U.S. consumers.

What supporters say: TransCanada, which is building the pipeline, says it would provide a safe, secure and reliable source of energy. Supporters also say it would cut U.S. dependence on Mideast oil and offer a multitude of jobs and tax benefits.

What you might remember: It took six years of review before President Obama announced in November 2015 that his administration rejected the project. "The State Department has decided that the Keystone XL pipeline would not serve the national interest of the United States. I agree with that decision," Obama said at the time. Two months earlier, Democratic candidate Hillary Clinton — who had maintained a long silence on the project under mounting pressure — also announced her opposition.